1. Field of the Invention
The present invention relates to a digital communication system, and more particularly, to a vestigial sideband (VSB) transmission system.
2. Background of the Related Art
The Advanced Television Systems Committee (ATSC) has selected the 8 Trellis-Vestigial Sideband (8T-VSB) modulation method as a standard in 1995 for the U.S. digital terrestrial television broadcasting, and the actual broadcasting incorporating the method has started since the second half of the year 1998. FIG. 1 illustrates an existing ATSC 8T-VSB transmitting system. It includes a data randomizer 1, a Read-Solomon encoder 2, a data interleaver 3, a trellis encoder 4, a multiplexer 5, a pilot inserter 6, a Vestigial Sideband (VSB) modulator 7, a radio frequency (RF) converter 8, and an antenna 9. First of all, the data randomizer 1 randomizes the input data received and outputs the randomized data to the Read-Solomon encoder 2. Then the Read-Solomon encoder 2 encodes (Read-Solomon encoding) the randomized data and adds a 20 bytes parity code. Next, the data interleaver 3 interleaves the data, and the trellis encoder 4 converts the interleaved data into symbols and performs trellis encoding. After the multiplexer 5 multiplexes the trellis encoded data and the sync signals, the pilot inserter 6 adds a pilot signal to the multiplexed symbols. Thereafter, the VSB modulator 7 modulates the symbols to 8T-VSB signals and outputs them to the RF converter 8. Finally, the RF converter 8 converts the 8T-VSB signals into RF signals, and the RF signals get transmitted to a receiving system.
FIG. 2 illustrates an existing ATSC 8T-VSB receiving system. It includes a demodulator 11, a comb filter 12, a slicer predictor 14, a channel equalizer 13, a phase tracker 15, a trellis decoder 16, a data deinterleaver 17, a Read-Solomon decoder 18, and a data derandomizer 19. Initially, the demodulator 11 converts the RF signals received through an antenna 10 into baseband signals. Then the comb filter 12 eliminates the NTSC interference signals from the baseband signals, and the channel equalizer 13 compensates the distorted channels using the slicer predictor 14. Thereafter, the phase tracker 15 tracks the phases of the received signals. Next, the trellis decoder 16 decodes the trellis encoded signals, and the data deinterleaver 17 deinterleaves the output of the trellis decoder 16. Finally, the Read-Solomon decoder 18 decodes the deinterleaved data, and the derandomizer 19 derandomizes the decoded signals.
It is very important to note that the ATSC 8T-VSB receiver can receive only MPEG (Moving Picture Experts Group) data, but not any other additional data such as program execution files or certificate information. In other words, the ATSC 8T-VSB transmission/receiving systems are only for MPEG image or sound data. In order to satisfy various demands of many users (viewers), the system should be able to send or receive various supplemental information as well as the image/sound data through digital broadcasting channels.
In addition, it is expected that some of the users will have to use a portable device or a personal computer (PC) card attached with a simple antenna in order to receive the supplemental data. In a case when a system receives data in a room, the data reception performance is generally poor due to the noises and ghosts resulted from reflected waves and many others. It is important to note that the supplemental data transmission must have a lower error rate compared to the image/sound data transmission. That's because even one bit error can create a very serious problem. Therefore, the system must be able to overcome the ghosts and noises generated in the channel.
Normally, the supplemental data will be transmitted together with the MPEG image/sound data using a time-sharing method. However, many ATSC VSB digital broadcasting receivers that can receive only the MPEG data are already on the market. Therefore, the supplemental data subjected to be transmitted with the MPEG data should not have any effect on the old receivers for receiving the MPEG data. In other words, the supplemental data broadcasting system should be compatible with the existing ATSC VSB systems.